Human breast cancers frequently trigger immune recognition and yet somehow evade immune rejection. We hypothesize that a significant component of immune evasion by breast tumors involves the exclusion, deletion, or functional impairment of tumor- specific lymphocytes after antigen recognition has occurred. Understanding mechanisms of immune evasion may lead to effective counter-strategies to prevent or reverse this process in human breast cancer patients, thereby restoring anti-tumor immunity. To test this hypothesis and develop a system for studying immune evasion in vivo, a new murirte model for breast cancer is proposed in which the response of naive, tumor-specific CD4+ and CD8+ T cell clones to mammary tumors can be monitored over time and after various immune interventions. Two epitopes, called OT1 and OT2, will be attached to the neu oncogene (aka HER2, erbB-2), so as to "epitope tag" neu for recognition by currently available CD8+ and CD4+ epitope-specific, TCR-transgenic T cell clones. The resulting chimeric protein, termed newOT1/OT2, will be tested in vitro for the retention of signaling and transforming ability and for recognition by OT1- and OT2-specific T cell clones. neuOT1/OT2 will then be expressed as a transgene in the mammary epithelium of C57BI/6 mice using the promoter from mouse mammary tumor virus (MMTV). As controls, mice expressing untagged neu will also be generated. Founder lines will be identified that develop metastatic mammary adenocarcinomas which can be recognized by OT1- and OT2-specific T cells. In future, naive OT1 (CD8+) and OT2 (CD4+) T cells will be infused into tumor- bearing mice to determine whether tumor-specific T cells can infiltrate mammary tumors and whether this induces T cell proliferation, anergy or apoptosis. Synergistic interactions between the CD4+ and CD8+ T cells will also be investigated. These studies will identify the levels at which mammary tumors evade recognition and/or rejection by both the CD4+ and CD8+ T cell subsets, which will lead to further investigation of the underlying molecular mechanisms. The long-term goal is to define the critical parameters that promote and inhibit effective T cell responses against mammary tumors, and to use this information to develop improved immunotherapies for breast cancer patients.